Micro-Electromechanical System (MEMS) that use electrical signals to move micromechanical structures are well known. Examples of devices using such MEMS include spatial light modulators (SLMs). SLMs use electrostatic forces between movable structures or actuators and base electrodes in or on an underlying substrate to actuate or move the movable structures thereby modulating the light incident thereon. The incident light beam can be modulated in intensity, phase, polarization or direction.
Generally, the movable structures, or membrane, are deflected towards the base electrodes by electrostatic forces when a voltage is applied between the movable structures and the base electrodes by drive circuits formed in or on the surface of the substrate adjacent to the MEMS.
Although an improvement over prior art non-MEMS based devices, conventional MEMS devices, such as SLMs, using voltage control drive circuits or drivers are not wholly satisfactory for a number of reasons. These reasons include the relatively large amount of space or surface area on the substrate used for lines connecting each of the base electrodes to a high voltage (HV) source, usually located off the chip or integrated circuit (IC) on which the MEMS is fabricated. This is especially problematic for the latest generation of high resolution SLMs having a two-dimensional (2-D) array of MEMS arranged in a compact manner that makes it difficult to provide the necessary HV lines to the base electrode for each of the MEMS. Moreover, transistors on each of these lines to control the HV are typically much larger than the transistors commonly used in ICs, making it difficult if not impossible to integrate the required drive circuitry on the same chip or IC as the SLM.
Another problem that arises with conventional voltage control drivers in a “snap-down” effect in which the electrostatic attraction causes the movable structure to deflect beyond the point when the restoring force is larger than the electrostatic force. Then the electrostatic force takes over and drives the piston hard into the substrate. This is always a problem for electrostatically operated SLMs and similar MEMs. Moreover, once snap-down has occurred van Der Waal forces can cause the movable structure to adhere or stick to the surface of the substrate or base electrode, rendering the SLM inoperable for a time if not indefinitely. Even in situations where the movable structure does not stick, the strong attractive forces generated by the proximity of the electrodes having a large difference in potential in a snap-down condition can lead to an undesirable transfer of material from the electrode to the substrate. Over time, this leads to catastrophic failure.
Accordingly, there is a need for a drive circuit or driver for MEMS devices that eliminates the need for numerous HV lines and transistors coupled to base electrodes of each MEMS, thereby reducing the size and complexity of the driver and enabling the use of compact 2-D arrays of MEMS. It is further desirable that the driver eliminates or greatly reduces the incidence of the snap-down effect that can occur with conventional voltage controlled drivers and provide for a larger design margin, thereby increasing the reliability of MEMS devices driven by the drive circuit and improving its' response time and linearity.
The present invention provides a solution to these and other problems, and offers further advantages over conventional voltage controlled drivers (such as a lower operating voltage).